The adoption of Orthogonal Frequency Division Multiplexing (OFDM) for transmitters in communication devices has reduced the efficiency of the linear power amplifiers (PAs), which causes a significant amount of the total energy consumption. This is due to the high peak-to-average power ratio (PAR) of the transmitted signal that OFDM results in. A means for improving the efficiency is to let the PA power supply follow the envelope of the transmitted signal. By doing this, the PA does not use more bias current than needed, regardless if the transmitted signal has a high or low momentary power.
Tracking the envelope of the transmitted signal requires a supply modulator which has sufficient bandwidth, in order not to distort the output signal, and high efficiency, in order not to loose what is gained in the improved PA efficiency. The two properties mentioned are difficult to combine as the switched voltage regulators (step-down (or buck)/step-up (or boost) DC-DC converters) often used, as depicted in FIG. 1, suffer from switching losses if they are switched quickly, which is required if the bandwidth is high.
The article Patrick Y. Wu et al “A two-phase switching hybrid supply modulator for RF power amplifiers with 9% efficiency improvement”, IEEE J. Solid-State Circuits, vol. 45, no. 12, pp. 2543-2556, December 2010, describes a two-phase switching hybrid modulator, as depicted in FIG. 2, consisting of one slow, highly efficient buck converter supported by a linear modulator which has a high bandwidth. However, the linear modulator normally suffers from very poor efficiency which reduces the actual overall efficiency gained from tracking the envelope.
EP2493060A1 discloses a multi-level supply modulator, or Step-Up/Down converter, as depicted in FIG. 3. This multi-level supply modulator comprises a multi-level charge pump. Furthermore, this multi-level supply modulator comprises an LC-filter (L, COUT). Such a multi-level supply modulator is theoretically more efficient (than the hybrid modulator described above) as it does not use any linear modulator. However, of complexity reasons, the number of levels is limited. Therefore the switching speed still needs to be relatively high, which causes losses as described earlier. The losses occur when the gates of the switch transistors need to be charged and discharged while switched on and off. The losses become even more significant when large switches have to be used to carry large currents.